If you don't have access to poke around in the registry (happens to me often) a poor-mans method would be to look for the system environment variable ARCHOME. If that's there, check under that for the existence of the executable file ARCHOME\bin\arc.exe (or, on Unix, $ARCHOME\bin\arc.exe).

Because the default "Program Files" folder contains spaces, you are more likely to find ARCHOME at somewhere like : "C:\arcgis\arcexe10x".

ARCHOME is required to be set for ArcInfo Workstation to operate correctly, and it will be removed from the environment on uninstall.

Here's a VBS fragment I used for uninstalling Google Earth via a startup script. You can use these WMI objects to poll machines for the ArcInfo Workstation installation, assuming it's registered in the product catalogue.